Combustion of various coals results in sulfur dioxide emissions in excess of governmental standards. Alkali impregnation of coal has been shown to be an inexpensive approach to reducing the SO.sub.2 emissions from combustion of sulfur containing coal, and, under some conditions, may be economically competitive with stack gas scrubbing. CaO reacts with SO.sub.2 from oxidation of coal sulfur compounds, ultimately forming CaSO.sub.4 which is retained largely in the coal ash.
An alternate approach to the wet alkali coal impregnation technique is dry blending calcium containing materials, e.g., limestone, with coal before or during combustion. The commercial viability of this approach will depend in part on maximizing the SO.sub.2 capture efficiency of the additive. While the type and origin of the calcium-containing additive is known to be an important factor in determining SO.sub.2 capture efficiency, the effectiveness of the best calcium-containing additive has not been sufficient to reduce the SO.sub.2 emissions to governmental requirements at practical loadings of limestone. Accordingly, a need has existed for improving the capture efficiency of calcium containing materials in coal combustion methods. The invention satisfies that need.